1. Field of the Invention
The invention generally relates to a method and apparatus for determining the radiation concentration level, air change rate and radiation source emanation magnitude for Radon (Rn-222) and Thoron (Rn-220) and their progenies in closed spaces.
2. Brief Description of Prior Art
In regions around the world, radon and thoron are present in buildings at concentration levels sufficient to be hazardous to the general health of building occupants. As is well known, radon and thoron seep up naturally from underground deposits in the form of a gas. The U.S. Environmental Protection Agency (EPA) has set out standards for permissible concentration levels of those gases, due to danger of lung cancer and other adverse health effects. Generally speaking, the concentration of radon gas (for instance) in a building is a function of the natural emanation or source rate, and the ventilation (or air change) rate in that building.
However, no single instrument is readily available to conveniently and concurrently determine the concentration, air themselves. If a high concentration is observed, the EPA recommends that the concentration be reduced below EPA standards.
To measure the radon or thoron concentration itself, an example of a system for measuring the concentration level is Radon "Sniffer", Model TN-WL-01, made by Thomson-Nielsen Electronics, Ltd. of Ontario, Canada. That system is a portable electronic device which forces ambient air through a filter, extracting the solid progenies of radon and thoron. The filter is situated adjacent to a silicon-based alpha particle radiation detector which counts the alpha particles emitted from the filter that impinge on the detector surface. With proper calibration and knowledge of air flow rate, the concentration of the progenies in air can be manually calculated.
Another filtered apparatus like that described above is the Alpha CAM Model 758, made by Victoreen, Inc. of Cleveland, Ohio. That patent describes an apparatus which measures airflow rate through the filter, and automatically computes the airborne radiation concentration in units of pico-Curies per liter of air.
Once an undesirable radon concentration is detected by devices like those reflected in the above patents or otherwise, reduction is needed. One way to reduce radon concentration is to increase the air dilution rate in the subject space by increasing the mixing with or ventilation to outside, lower-radiation air, if at all economical. This can only be evaluated by knowing the current ventilation or air change rate (ACH, units of h.sup.-1). Concentration may also be reduced by inducing a positive pressure differential between the space and the Rn source, if ventilation rate is known. EPA Report 625/5-87/019, entitled "Radon Reduction Technique for Detached Houses" (incorporated here by reference) in fact indicates that the single most effective measure a homeowner can take to reduce radon concentration is to increase the ventilation rate.
The EPA Report further cites measurement of house ventilation rate as a diagnostic to help select radon reduction techniques. Even so, an informal survey of Rn testing/mitigation firms in the state of Maryland (involving 90% of licensed firms) found that only two firms had the capability, in-house, to measure ventilation rates.
The current standard method for determining ventilation or air change rate in a building space is to inject a tracer gas, typically SF.sub.6 (ethane may also be used) into the space. The decrease in the SF.sub.6 concentration with time as ventilation carries the SF.sub.6 away is monitored with a tracer gas monitoring instrument. One such instrument is the Leak Meter Model 61, manufactured by Ion Track Instruments, Inc. of Wilmington, Mass. However, this instrument requires ancillary equipment including high pressure tanks of argon and SF.sub.6 gas, which altogether is cumbersome and not easily handled in the field.
Besides increasing ventilation, a second way to reduce the radon concentration level below EPA-recommended limits is to reduce the radiation source (emanation) rate into the space by modifying the building itself, to seal off or reduce radon and thoron entry pathways. This usually involves expensive physical revisions to the building. Therefore, to establish whether the option of reducing the source rate magnitude is preferable to the option of increasing the air change rate in the space, it is necessary to determine both the air change rate and the radiation source rate magnitudes.
But again, no single apparatus is available that can measure the airborne radon (or thoron) concentration level, and simultaneously determine the air change rate and the source rate magnitude. Moreover, no single instrument can determine just the radiation source rate magnitude.
Thus, there is a technical need for a radon and thoron radiation monitoring instrument that can also measure the air change rate, and determine the radiation source rate magnitude. There is also a need for an instrument that is compact, portable, easy to transport to and operate at field testing sites.